Cleavage is the process of rapid, synchronous cell divisions that occur after fertilization and before the mid-blastula transition. It involves the division of the zygote into increasingly smaller blastomeres through mitosis without cell growth. The planes and patterns of cleavage are determined by the distribution of yolk and cytoplasmic movements in the egg. This results in either complete or partial cleavage, forming a hollow ball of cells called a blastula. The transition to asynchronous cell cycles and zygotic transcription marks the mid-blastula stage.
2. • After fertilization, the development of a multicellular
organism proceeds by a process called cleavage, a series
of mitotic divisions whereby the enormous volume of egg
cytoplasm is divided into numerous smaller, nucleated
cells.
• Cleavage is a series of rapid cell divisions without cell
growth or gene expression which occurs in early
embryogenesis.
• The cleavage can be defined as the process of
progressive subdivisions of the zygote by mitotic cell
division into an increasing number of cells of
progressively decreasing size.
• The consecutive divisions of cells results in heap of cells
called morula
3. • The cleavage stage cells are called
blastomeres. The total cellular volume of
the embryo stays the same, but the
number of cells within the embryo
increases. The cleavage rhythms are
controlled by maternal factors present in
the cytoplasm of ovum.
• The cell cycle is biphasic. Only M and S
phase. Mitosis in cleavage stage of
Drosophila occurs every 10 min. 50000
cells are produced in 12 hrs.
4. • Transition from fertilization to cleavage is because of
activation of MPF.
• MPF activity is highest during M phase.
• Undetectable in S phase.
MPF - mitotic promoting factor- complex* of two proteins: cdk + cyclin
MPF is a kinase enzyme, switches on/off target cell cycle proteins by
phosphorylating them.....
inactive cycle protein ------------->active-P
ATP ADP
MPF promotes entrance into mitosis from the G2 phase by phosphorylating
multiple proteins during mitosis including one that leads to destruction of cyclin
itself
MPF cdk - a cell division control protein - cyclin dependent kinase;
active only when bound to cyclin;
cyclin - a protein whose amount varies cyclically*;
when in high concentrations*, binds to cdk makes MPF...
[cyclin + cdk = MPF].. favors Mitosis GF
regulated at critical points... Cell cycle checkpoints*
5. • Cleavage is actually the result of two
coordinated processes. The first of these
cyclic processes is karyokinesis the
mitotic division of the nucleus. The second
process is cytokinesis the division of the
cell.
6. Planes of cleavage
The asters of mitotic spindle determine the plane
of cleavage.
Meridional plane: The cleavage takes
place from animal pole to vegetal pole
passing exactly from the center of the
animal-vegetal axis. Bisects the zygote.
First cleavage furrow of Amphioxus
and frog
Vertical plane: The cleavage takes place
from animal pole to vegetal pole passing
away from the center of the egg on both
sides.
3rd and 4th cleavage furrows of Chick and
frog
7. Equatorial plane: the cleavage furrow
passes from the equator of the
egg.
3rd cleavage of Amphioxus
Latitudinal/Transverse/Horizontal
plane: the cleavage furrow passes
from both the sides of equatorial
plane
5th cleavage of Amphioxus and frog
Planes of cleavage plays important
role in determining the form of the
embryo. The yolk distribution will
also decide the location of the
spindle
8. Patterns of the cleavage
• Cleavage has a special relationship to the egg
regions established by the cytoplasmic
movements. The position of the first cleavage is
not random, but tends to be specified by the
point of sperm entry and the subsequent rotation
of the egg cytoplasm. The coordination of
cleavage plane and cytoplasmic rearrangements
is probably mediated through the microtubules
of the sperm aster.
9. • Complete Cleavage: Holoblastic
The cleavage plane divides the egg
completely. It is further divided into
Bilateral, Radial, Rotational and Spiral.
Bilateral: The first cleavage results in
bisection of the zygote into left and right
halve. The following cleavage planes are
centered on this axis. The two halves of the
embryo are mirror images of each other.
11. • Radial Cleavage: In this pattern when
organism is viewed from above (Dorsal or
animal pole) is essentially radial in
symmetry. Characteristic of
Deuterostomes (some vertebrate and
echinoderms. The spindle axes are
parallel or right angles to the polar axis.
12. • Spiral Cleavage: First two divisions result
in megameres such as A,B,C and D. Each
blastomere represents one quadrant of the
embryo. A and C macromeres meet at the
animal pole while B and D meet at the
vegtal pole. The divisions are at the
oblique angle.. Each micromere is rotated
relative to their parent macromere. The
rotation may be clockwise ot
counterclockwise.
13.
14. • Complete Cleavage: Depending on the
distribution of yolk cleavage is further
divided into
• Holoblastic equal cleavage:
• When the amount of yolk is less or
moderate and distribution of the yolk is
even. The cleavage furrow divides the egg
completely into equal sized blastomeres.
These blastomeres lie one above the
other. The resulting cleavage is holoblastic
equal. Eg Amphioxus
15. • Holoblastic unequal Cleavage:
• The amount of yolk is large. The yolk is not
distributed evenly. Animal pole will have less
yolk and more cytoplas. On the other hand
vegetal pole will have more concentration of
yolk. The cleavage furrow divides the egg
completely. The blastomeres formed are of
unequal size. The animal pole will have
micromere (small in size), while vegetal pole will
have megameres(large in size). The micromeres
divide at a faster rate compared to megameres.
• Eg Frog .
16. • Meroblastic cleavage: in the eggs having
large amount of yolk partial cleavage is
observed. The egg is not cleaved
completely.
• Meroblastic discoidal cleavage:
• Cleavage furrow do not penetrate the yolk.
The embryo forms the disc of the cells at
animal pole where the cytoplasm is
present. The disc is formed at the top of
the yolk called blstodisc.
17.
18. • Meroblastic superficial Cleavage:
• In this type of cleavage nuclei divide by
mitosis but there is no cytokinesis. Many
nuclei are present in the single cell forming
syncytium. The yolk is present in the
center of the egg. The nuclei migrate at
the periphery. The plasma membrane
grows inwards for partitioning the nuclei
into individual cells. The yolk is in the
centre and these blastomeres form a layer
around the central yolk.
19. Determinate cleavage: In some animals the
blastomeres have a predetermined future. The
blastomeres are determined to give rise to specific
parts of the embryo. A mosaic type of blastula is
formed.
Eg Ascaris, Mollusca
Indeterminate cleavage: The fate of the blastomeres
is not so rigid. Blastomeres exhibit plasticity. At
two cell stage if the blastomeres are separated,
each one can produce complete embryo. The
future of the blastomeres is not pre determined.
Eg. Vertebrate Embryos.
20. • Successive cleavages result in the
formation of sheet of cells enclosing a
hollow fluid filled blastula.
• In sea urchin the cleavage is radial, the
cells remain in a single layer surrounding
the cavity blastocoel.
24. • What leads up to the mid-blastula
transition?
• Fertilization
• 1. Sperm & egg recognition
• 2. Sperm entry
• 3. Fusion of genetic material
• 4. Activation of egg metabolism
